The first scanning probe microscope was the Nobel prize winning scanning tunneling microscope (STM) of Binnig and Rohrer described for instance in U.S. Pat. No. 4,343,993. Since this original design, a whole family of scanning probe microscopes has grown up. Notable members are the AFM in which the atomic force between tip and sample is measured as opposed to the tunneling current in the STM; the NSOM, described for instance in EP-A-0112401, in which a waveguide tip, usually the tip of an optical fiber, measures optical coupling between tip and sample; and the FFM in which a force arising from coupling between tip and sample due to the presence of a liquid or gaseous viscous medium such as water or air respectively.
The present invention relates to scanning probe microscopes of the type in which the tip is vibratable relative to the sample. Typically, the tip or sample is set into periodic motion by a driver means, omen referred to in the art as a dither and sometimes as a wobbler. The tip-sample coupling is measured by its effect on one or more of the vibrational properties of the tip. Known methods of measuring the tip-sample interaction are through changes in the frequency of vibration of the tip, changes in the amplitude of vibration of the tip, and changes in the phase of vibration of the tip.
In U.S. Pat. No. 4,851,671 the frequency of the vibration of the tip is measured by electrical means. The tip is secured to a piezoelectrical crystal which is driven at one of its natural resonance frequencies, thus setting the tip into oscillatory motion normal to the surface of the sample by excitation of a shear mode of the piezoelectric crystal. Tip-sample interaction changes the frequency of vibration of the tip and results in feedback into the driver circuit. This frequency change of the driver circuit is then measured with standard electronic counting means.
Toledo-Crow et al disclose a scanning probe microscope in Applied Physics Letters, volume 60, pages 2957 to 2959 (1992) which measures changes in the amplitude of the tip vibration. This has the advantage of offering an intrinsically more rapid measurement than measurement of the frequency since alterations in the damping, i.e. the magnitude of the vibration can, in principle, be measured instantaneously. Toledo-Crow et al use optical means to measure the vibrational amplitude. This optical means comprises a laser source, a Wollaston prism, a beam splitter, an objective lens, a polarisation analyser and a light detector. I have built an apparatus following the design of Toledo-Crow et al and have found it to be accurate, rapid and quite sensitive.
However, despite these advantages, it is quite complicated to build, it is costly, it takes up a considerable amount of space and also requires alignment of the optical components. Additionally this alignment can be disturbed by mechanical shocks. Due to the alignment required, it is difficult to operate such an apparatus in a completely automated manner as required for instance in a satellite, in a hazardous environment such as a nuclear reactor, in a vacuum chamber, or in a cryostat and as desired in a commercial turnkey system. The spatial requirements of the apparatus can also be a problem in some applications, for instance it would not be easy to design an apparatus of this kind for use in the limited sample space of a magnet cryostat. A further consequence of the size and alignment requirements, is that, in order to build up an image, it is the sample which must be tastered since rastering the tip would necessitate rastering the whole optical set-up which would be impractical. This is not important for some samples, but can be a problem for large or heavy samples, such as mechanical work pieces, or samples which cannot be kept still, such as living organisms or plant matter.
Another apparatus which uses optical means to measure the vibration of the tip is described by Betzig et al in Applied Physics Letters, volume 60, pages 2484 to 2486 (1992). Embodiments are disclosed which not only measure the amplitude and/or phase of the vibration. The apparatus of Betzig et al has similar advantages and disadvantages to that of Toledo-Crow et al.
It has thus been recognized in the art that the optical external deflection sensors which are particularly prevalent in cantilever AFM designs (see for instance EP-A-0 422 548 and EP-A-0 394 962) work very well but, when used, make up a large part of the complexity, size and cost of the instrument. There is therefore a recognised want for simpler, less costly sensing means with at least comparable sensitivity. In particular, the piezoelectric effect has been used, not only in the above-mentioned U.S. Pat. No. 4,851,671 but also by Tortonese et al in Applied Physics Letters, volume 62, pages 834 to 836 (1993) and by Tansock and Williams in Ultramicroscopy, volume 42 to 44, pages 1464 to 1469 (1992).
Tortonese et al use a piezoresistive effect whereby the cantilever arm of an AFM is made of piezoelectric material and bending of the cantilever arm by the tip-sample interaction changes the resistance of the cantilever arm. This approach has proved to work well but the sensor is restricted to DC operation since there is no inverse piezoresistive effect, i.e. one cannot apply a resistance to strain the cantilever arm. Consequently, this approach is not suitable for scanning probe microscopes of the initially named kind.
Tansock and Williams describe a cantilever suitable for an AFM in which the cantilever is also made of piezoelectric material but in the form of a bimorph. This cantilever is therefore vibratable and hence suitable for use in a scanning probe microscope of the initially named kind by applying an AC voltage across either half of the bimorph.
However, the Q-factor of a simple cantilever, i.e. of a single beam, is typically poor, having for example a value of only Q=7 in the above publication.
Dransfeld et al in U.S. Pat. No. 5,212,987 disclose an acoustic scanning microscope using a piezoelectric tuning fork. The tuning fork is driven so that the oscillation of its prongs generate acoustic waves. The tuning fork needs to be aligned relative to the sample to be measured such that the direction of oscillation of the prongs is inclined either perpendicular to or with a significant component perpendicular to the sample surface. Acoustic waves then pass through the air, or another fluid medium which can support acoustic waves, and reflect from the sample surface and then return to the tuning fork and cause feedback. In this way the oscillations of the tuning fork are sensitive to the topography and acoustic properties of the sample surface and an image can be built up.
The acoustic scanning microscope also has the restriction that the signal is highly dependent on the acoustic medium, there being for example a strong pressure dependency of the characteristic. In vacuum the method cannot work at all as no acoustic waves are supported. In liquid mediums such as liquid helium it is not clear whether such a technique would be practical and what effects, for instance, the superfluid phase transitions would have. Moreover, due to the wavelength of acoustic waves, the technique is limited to a best resolution across the sample surface of approximately 50 nanometers.
It is thus an object of the present invention to provide a scanning probe microscope, wherein changes in the vibration of the tip are measurable by means which do not require optical or mechanical alignment, wherein changes in the vibrational state of the tip can be made rapidly, accurately and to a high degree of sensitivity, wherein the probe head is compact and light, wherein the probe head is rugged, wherein the SPM is operable in hostile and confined environments, such as in cryostats or in a vacuum chamber, wherein the SPM is inexpensive relative to prior art SPMs of comparable performance, and/or wherein the Q-factor associated with the pick-up means of the SPM is easily selectable during manufacture to a desired value in the range of approximately Q=10.sup.2 to Q=10.sup.6.